1. Field of the Invention
The present invention relates to a high temperature method of bonding substrates made of a metal or an alloy.
2. Description of the Prior Art
Various bonding methods including welding methods and brazing methods have been known as methods of bonding substrates made of a metal or an alloy.
The method of bonding substrates made of a metal or an alloy at a temperature lower than a melting point of the metal or the alloy are classified into a solid phase bonding method as various welding methods and a brazing method.
In the solid phase bonding method, clean surfaces of substrates made of a metal or an alloy are contacted and closely contacted to atomic distance for imparting cohesive force between atoms. Thus, it is not easy to attain such condition.
The solid phase bonding method is classified into a deformation bonding and a diffusion bonding depending upon each combination of a temperature and a pressure.
The deformation bonding method is to bond mainly by a slip deformation and twinning deformation and it is attained at relatively lower temperature for relatively shorter time under relatively high pressure to give relatively greater deformation.
The diffusion bonding method includes most of the conventional high pressure methods such as gas pressure welding method, contact resistance welding method, ambient temperature pressure welding method, ultrasonic welding method, friction welding method and explosion welding method wherein a diffusion of atoms and a creep deformation are mainly utilized to contact closely the contacted surfaces to bond at relatively high temperature for relatively longer time under relatively lower pressure to give less deformation.
The diffusion bonding method also includes a method of bonding substrates by inserting an insert metal at the bonding part beside the direct welding methods.
The insert metal is usually a metal having high diffusion velocity and a melting point lower than that of a substrate. It is clear that the insert metal is used for improving close contact of surfaces by utilizing its softness and for accelerating the bonding by utilizing its thermal diffusing characteristic, and for reinforcing the bonded surface by forming an alloy, and for increasing a remelting temperature.
The diffusion bonding method is attained under one kind of surface phenomenon and accordingly, the surface condition is important factor.
In order to attain the close contact for a large area under relatively lower pressure and relatively less deformation, the thermal diffusion and the creep deformation characteristics are especially important for bonding them.
On the other hand, according to the definition of International Standaring Organization, a brazing method is defined to a method of bonding substrates by using a molten metal (braze) under utilizing wetting and spreading characteristics on the substrates wherein a melting point of the molten metal is lower than that of the substrate and a brazing is to use a molten metal having a melting point of higher than 450.degree. C. and a soldering is to use a molten metal having a melting point of lower than 450.degree. C., and the melting of the surfaces of the substrates does not constitute the joint.
The meaning of the last definition is considered that the joint for the bonding is not formed by the molten substrates but is formed by a braze. Thus, in the high temperature brazing method for heat resistant alloy, an alloy of the substrate is formed. The definition may be modified.
The disadvantages of the conventional diffusion bonding methods are to require special clean surfaces and smoothness in comparison with the welding methods and the other solid phase bonding methods.
In order to contact closely the surfaces of the substrates, it is necessary to reduce roughness and unevenness of the surfaces of the substrates. The substrates can be bonded at lower temperature under lower pressure depending upon the cleanness and smoothness of the surfaces of the substrates. It is reported to be preferably processing the surface having a roughness of less than about 10.
The surface treatment is usually carried out by a wire brushing or an acid washing. Recently, it has been proposed to treat by glow discharge in an inert gas under a reduced pressure or by an ion plating. The surface treatment should be carried out just before the bonding operation and the bonding characteristic is deteriorated depending upon an aging after the surface treatment. The treated surface should be usually bonded in vacuum, an inert gas or a reducible atmosphere in order to prevent the inactivation of the treated surface.
The other disadvantage of the conventional diffusion welding method is to require relatively high temperature such as about 0.7 T.sub.M (T.sub.M : melting point (K.degree.)). The temperature in the welding highly affects to a creep deformation, a diffusion velocity, a dissociation of an oxide and a dissolution of an oxide into the substrate, whereby the temperature is important factor in the diffusion welding method.
As experimental facts, the minimum temperature to attain easy bonding in the solid phase bonding of a metal is about a recrystallizing temperature and the bonding is easily attained at higher than the minimum temperature.
The conventional diffusion bonding is to attain a precise welding whereby it is necessary to prevent a deformation of the substrate and to minimize the pressure. However, the pressure affects to break a surface film and to improve close contact and to promote a diffusion of atoms and a creep deformation. The contradiction to the theory is compensated by the special surface treatment.
A bonding method similar to the brazing method can be considered to overcome the disadvantages of the conventional diffusion bonding methods. Such method is called as Transient Liquid Phase Bonding Method (TLP method) wherein an insert metal having a thickness of less than 0.1 mm and a low melting point is used and the substrates are bonded under light pressure in vacuum or argon gas by heating them. The insert metal is melted at the bonding temperature to fill the gap of the bonding boundary with a thin liquid layer. During the maintenance to the bonding temperature, a mutual diffusion is caused between the liquid insert metal and the substrate and the components in the bonded part are changed by causing an isothermal solidification to bond them. The disadvantage of the TLP method is to melt mutually the liquid insert metal and the substrate at relatively low temperature for a long time.
On the other hand, the disadvantage of the conventional brazing method is to form a strong surface film on the surface of the substrate made of an active metal such as Al, Cr and W or an alloy thereof whereby the bonding property is significantly deteriorated.